Photodetectors are used to detect light of a given wavelength and produce a current proportional to the intensity of the detected light. A photodetector may be supplied with a bias voltage. The output of the photodetector may vary with variations of the bias voltage.
A prior art photodetector circuit 100 is shown in FIG. 1. The circuit includes a photodetector 150 between a high voltage supply 110 and a transimpedance amplifier (TIA) 180 including an amplifier 140 and a resistor 130. The signal (photocurrent) from the photodetector 150 is amplified by the transimpedance amplifier 180, thereby converting a photocurrent from the detector into an output voltage 160 suitable for post processing, for example, by a post processing circuit (not shown).
In the prior art photodetector circuit 100, the transimpedance amplifier 180 may be used to convert the photocurrent from the photodetector 150 into a voltage for further processing. Such circuits are widely used in various applications such as optical communication or laser range finding. In some applications, for example, communication applications, the average photocurrent is fairly constant and can easily be handled by the prior art transimpedance amplifier 180.
However, range finding applications behave differently in that the light signal level can be very low, for example, almost zero, followed by a strong optical pulse having a high, short duration signal level. When such a high power pulse impinges the detector 150, a large photocurrent can flow through the photodetector 150. This photocurrent may only saturate the optical receiver of the circuit 100, but may also damage both the photodetector 150 and transimpedance amplifier 180. Protection of the transimpedance amplifier 180 and photodetector 150 is thus very important.
It can take a while for the transimpedance amplifier 180 to return to pre-pulse condition after it has saturated. In normal operation the input impedance of the transimpedance amplifier 180 is approximately equivalent to the feedback resistance 130 divided by the voltage gain, Av, of the amplifier 140.
When the amplifier 140 saturates, the input impedance of the transimpedance amplifier 180 becomes approximately equal to the feedback resistance 130 in parallel with all parasitic capacitance including junction capacitance of the photodetector 150. The time required for the transimpedance amplifier 180 to recover is related to the resistance/capacitance (RC) time constant of the parasitic capacitance and feedback resistance 130 as well has the input voltage reached during saturation. The higher the photocurrent above saturation, the longer it takes to recover. This is referred to as the “depth of saturation” of the transimpedance amplifier 180.
Several strategies have been attempted to improve recovery time by reducing the equivalent input impedance during saturation. Such strategies include use of variable feedback impedance, variable shunt input impedance or input current sinking.
Variable feedback typically adds capacitance in the feedback path impacting the frequency response and noise of the optical receiver. Input current sinking is typically used to cancel low frequency input current. Most such strategies use some kind of feedback from the output of the transimpedance amplifier to react to the high photocurrent. This forces the user to reduce the bandwidth (frequency response) of the feedback in order to maintain the high frequency behavior of the receiver. However, in the case of high optical power pulse, it is not possible to use the output of the transimpedance amplifier 180 to apply protection since damage to the transimpedance amplifier 180 is likely to occur before the output can begin to respond to the signal. It is preferable to not rely on the output of the transimpedance amplifier 180 to improve recovery time.
Variable shunt impedance at input may include a diode for example reversed bias in normal operation and forward bias during saturation providing a low impedance. Recovery time is improved by limiting the input voltage reached during saturation but is dependent on the photocurrent from the photodetector.
Another technique to reduce the impact of a high power optical pulse is the use of a resistance in series with an avalanche photodiode (APD). This technique is useful for low frequency signals but not for high frequency signals or for signals in the presence of a strong optical power pulse since the series resistance impacts the bandwidth of the receiver, and if a decoupling capacitor is used to maintain the voltage after the resistance, the photocurrent from the photodetector is maintained as long as the decoupling capacitor can provide charges to the photodetector 150. Therefore, there is a need in the industry to address the above shortcomings.